The invention relates to an attachment device for fixing an aircraft engine to a strut secured to a structural element of the aircraft, such as a fuselage or wing element.
The attachment device according to the invention can be used on any type of aircraft. A preferred application relates to aircraft of modern design, whose engines are equipped with very large diameter fans.
Engines equipping aircraft are either suspended to a strut fixed beneath the aircraft wing, or are laterally attached to a strut fixed to the aircraft fuselage. In both cases the link between the engine and the strut is generally provided by a front attachment device and a rear attachment device. The function of these attachment devices is to transmit to the aircraft, by means of the strut, the forces produced by the engine.
To analyze the forces transmitted by the attachment devices, to the engine is allocated an orthonormal fix OXYZ, in which the OX axis, which is directed forwards is oriented in accordance with the longitudinal axis of the engine, the OY axis is oriented laterally in a horizontal plane and the OZ axis is oriented vertically upwards.
With respect to said fix, in the case of an engine suspended on the wing, the forces transmitted to the aircraft structure by the attachment devices are mainly the thrust forces of the engine, essentially applied in accordance with the OX axis, lateral forces due more particularly to a gust of wind, applied substantially in accordance with the OY axis and forces due to the weight of the engine or generated in the case of the aircraft crashing and essentially applied in accordance with the OZ axis. Moreover, attachment devices transmit to the aircraft structure a moment along the OX axis and which is due to the rotary movement of the engine. In the case of an engine mounted laterally on the fuselage, the forces applied in accordance with the OY and OZ axes are reversed.
To ensure the fixing of a turbojet engine to an aircraft attachment strut, there are at present two main fixture types, namely the xe2x80x9ccorexe2x80x9d type fixture and the xe2x80x9chybrid fanxe2x80x9d type fixture.
In the core type fixture, the central casing of the turbojet engine is fixed directly to the aircraft strut by a front mount and a rear mount. In this case, the front mount simultaneously absorbs the forces along the OX, OY and OZ axes and the rear mount absorbs the forces along the OY and OZ axes, as well as the moment along the OX axis.
In the case of a hybrid fan-type fixture, a front mount is interposed between the strut and the turbojet engine fan casing and a rear mount is interposed between the strut and the central casing of the turbojet engine, as in the case of the core-type fixture. In this case, the front mount absorbs the forces along the OY and OZ axes and the rear mount absorbs the forces along the OX, OY and OZ axes, as well as the moment along the OX axis. Moreover and as is more particularly illustrated by EP-A-564 126, the absorption of the thrust along the OX axis takes place by means of two rods linking the rear mount to the front part of the central casing of the engine.
Thus, in all conventionally used aircraft engine fixing systems, all the attachment devices other than the thrust absorption rods ensure the transmission of forces in at least two different directions.
Moreover, the thrust absorption rods are dimensioned so as to take up a considerable force, oriented in accordance with the longitudinal axis OX of the engine. Thus, they are long rods (approximately 2 m).
In addition and as is more particularly illustrated by U.S. Pat. No. 5,275,357, the attachment devices used in existing fixing systems usually incorporate an emergency attachment structure. This emergency attachment structure, which is directly connected to the strut or an intermediate fitting, is a specific part solely stressed in the case of a fracture of one or more elements of the main attachment structure. At present, all the parts of emergency attachment structures are passive in normal operation. They do not intervene when the main attachment structure is operational. They only become active in the case of the fracture of one or more parts of the main attachment structure. Thus, their absence would not deteriorate the transmission of forces across the main attachment structure of the attachment device.
Recent use of engines having ever increasing sizes and weights accentuates the problems such as the bending of the engine, vibrations, etc. In order to obviate such problems whilst ensuring a better weight distribution, the replacement of conventional fixing systems by a new system using three instead of two attachment devices is being envisaged. In this case, the strut would be connected to the fan stator case by a front attachment device only taking up forces applied in a single direction substantially corresponding to the vertical axis OZ when the engine is placed beneath the aircraft wing.
In order to implement this attachment device, it is not possible to use the two rod structure serving to take up the thrust forces in the hybrid fan-type fixture. Thus, the direction of the force to be transmitted is located in a vertical and no longer a longitudinal plane. Moreover, the attachment device must be as narrow as possible, so that the disturbance produced in the aerodynamic air flow is as small as possible. The reduced space available between the strut and the fan stator case of the engine makes it necessary to use a compact attachment device.
The attachment device having to fulfil this function must preferably be easily dismantlable, in order to avoid any time loss during the installation or removal of the engine.
Finally, it is no longer possible to make use of one of the attachment devices used in conventional fixing systems, such as the front mount of the hybrid fan-type fixture, which takes up or absorbs the forces exerted in accordance with the OY and OZ axes. Thus, due to the fact that existing attachment devices always take up forces exerted in accordance with at least one direction other than the OZ axis, their use makes it necessary to more extensively stiffen the strut in order to ensure a correct absorption of the forces transmitted by the attachment device. This involves adding material and consequently making the strut heavier, which is prejudicial. It would also be contrary to improving the performance characteristics of the engine, sought by the use of a novel fixing system including a front attachment device only taking up forces mainly applied along the vertical OZ axis.
The invention mainly relates to a simplified attachment device usable in a novel fixing system for an aircraft engine to an attachment strut, in order to ensure the transmission of forces in a single direction, in a reduced space and causing minimum disturbance to the aerodynamic air flow.
In secondary manner, the invention also relates to an attachment device which, if necessary, can easily be dismantled.
According to the invention, this result is obtained by means of a device for attachment an aircraft engine to a strut fixed to a structural element of the aircraft, said device comprising a main attachment structure and an emergency attachment structure, characterized in that the main attachment structure ensures the transmission of forces oriented towards the strut, in a single direction, which is substantially radial with respect to a longitudinal axis of the engine, the emergency attachment structure incorporating an element fixed to the strut, which also forms part of the main attachment structure.
Bearing in mind the essential function fulfilled in the attachment device according to the invention, the element common to the main attachment structure and to the emergency attachment structure advantageously comprises at least two separate parts able to independently transmit the forces oriented in said direction. Thus, in the case of the fracture of one of the two parts, the forces are still transmitted through the other part.
In a preferred embodiment of the invention, the main attachment structure also comprises a crossbar articulated substantially in its centre to the element common to the main attachment structure and to the emergency attachment structure, and two rods articulated to each of the ends of the crossbar, so as to connect the latter to an engine casing.
In this arrangement, the crossbar is preferably articulated to the element common to the main attachment structure and to the emergency attachment structure by a hinge pin oriented substantially parallel to the longitudinal axis of the engine.
The rods are advantageously articulated to the crossbar and to the engine casing by swivel pins. This arrangement avoids parasitic forces, i.e. forces oriented in a direction different to the aforementioned force transmission direction, do not pass through the attachment device.
In order to complete the emergency attachment structure, the element common to said structure and to the main attachment structure is advantageously connected to the casing by a connection having a clearance and which does not work when the main attachment structure is operational.
Advantageously, the connection with clearance comprises another hinge pin, mounted on the element by means of a ball joint and a hole formed in a cap integral with the casing and traversed with clearance by said other hinge pin.
In the case where the engine is a turbojet engine, the attachment device according to the invention is advantageously used for connecting the strut to the fan stator case of said turbojet engine.
Preferably, the element common to the main attachment structure and to the emergency attachment structure comprises a disconnection plane for fixing to the strut by fixing means.